This invention relates generally to techniques for isolating fuel cells from the external environment.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. A fuel cell catalytically reacts a fuel to create an electric current. The catalytic reaction causes electrons to be removed from the fuel elements. Fuel cells generally include an electrolyte material sandwiched between two electrodes called the anode and the cathode. Fuel passes over the anode and oxygen passes over the cathode. The fuel is catalytically split into ions and electrons. The electrons go through a external circuit to serve an electric load while the ions move through the electrolyte towards the oppositely charged electrode. At the electrode, the ions combine to create byproducts such as water and carbon dioxide.
Thus, fuel cells generally include a power section which has just been described and in addition may provide DC power to a power conditioner that outputs AC power. A fuel processor includes a reformer that receives the fuel such as natural gas and produces a hydrogen rich gas to the power section. While the electrochemical reaction in the fuel cell is similar to a battery, unlike the battery, which runs down, fuel cells continue to create electricity and thermal energy for so long as fuel and oxygen sources are supplied to sustain the reaction.
For example, one type of fuel cell includes a proton exchange membrane (PEM), that may permit only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the PEM. The electrons produced by this oxidation travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions may be described by the following equations: EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- at the anode of the cell, and O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O at the cathode of the cell.
Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several serially connected fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. A fuel cell stack 5 shown in FIG. 1 may include different plates 6 that are stacked one on top of the other in the appropriate order, and each plate may be associated with more than one fuel cell 7 of the stack 5. The plates 6 may be made from a graphite composite or metal material (as examples) and include various channels and orifices to, as examples, route the above-described reactants and products through the fuel cell stack. Several PEMs 8 (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. The anode 3 and the cathode 4 may each be made out of an electrically conductive gas diffusion material, such as a carbon cloth or paper material, for example. A rod 9 may connect the stack to a spring mounting foundation 2.
Referring to FIG. 2, as an example, a fuel cell stack 10 may be formed out of repeating units called plate modules 12. In this manner, each plate module 12 includes a set of composite plates that may form several fuel cells. For example, for the arrangement depicted in FIG. 2, an exemplary plate module 12a may be formed from a cathode cooler plate 14, a bipolar plate 16, a cathode cooler plate 18, an anode cooler plate 20, a bipolar plate 22 and an anode cooler plate 24 that are stacked from bottom to top in the listed order. The cooler plate functions as a heat exchanger by routing a coolant through flow channels in either the upper or lower surface of the cooler plate to remove heat from the plate module 12a. The surface of the cooler plate that is not used to route the coolant includes flow channels to route either hydrogen (for the anode cooler plates 20 and 24) or oxygen (for the cathode cooler plates 14 and 28) to an associated fuel cell. The bipolar plates 16 and 22 include flow channels on one surface (i.e., on the top or bottom surface) to route hydrogen to an associated fuel cell and flow channels on the opposing surface to route oxygen to another associated fuel cell. Due to this arrangement, each fuel cell may be formed in part from one bipolar plate and one cooler plate, as an example.
For example, one fuel cell of the plate module 12a may include an anode-PEM-cathode sandwich, called a membrane-electrode-assembly (MEA), that is located between the anode cooler plate 24 and the bipolar plate 22. In this manner, the upper surface of the bipolar plate 22 includes flow channels to route oxygen near the cathode of the MEA, and the lower surface of the anode cooler plate 24 includes flow channels to route hydrogen near the anode of the MEA.
As another example, another fuel cell of the plate module 12a may be formed from another MEA that is located between the bipolar plate 22 and the cathode cooler plate 20. The lower surface of the bipolar plate 22 includes flow channels to route hydrogen near the anode of the MEA, and the upper surface of the cathode cooler plate 24 includes flow channels to route oxygen near the cathode of the MEA. The other fuel cells of the plate module 12a may be formed in a similar manner.
In addition to the PEM type of fuel cells, there are a number of other types as well. Solid oxide fuel cells use ceramic solid phase electrolyte. A molten carbonate fuel cell uses a molten carbonate salt mixture as its electrolyte. Finally a phosphoric acid fuel cell uses liquid phosphoric acid as the electrolyte.
Generally, each type of fuel cell is subject to leakage. A number of fuel cells are adapted to home, vehicular or other portable uses. In these applications, leakage may be considered to be undesirable. For example, concentrations of gaseous fuel cell fuels such as hydrogen may be explosive. Fuel cell systems may also include other components that carry potentially explosive gasses, such as reforming systems that transform hydrocarbons, natural gas for example, into hydrogen. Fuel cells systems may also utilize hydrocarbon coolants that might pollute the environment if spilled. In addition, the transport of the fuel cell for assembly or for ultimate use may be complicated by leaking fluids, including liquids and gasses.
Thus, there is a continuing need for better ways to control the leakage of fluids from fuel cells.